Multimode optical fibers are used in a large number of applications, such as communications networks, sensors systems, avionic and aerospace industry, medical instruments, fiber bundles, and fiber amplifiers and lasers. One of the basic components in most of these applications is the multimode fiber coupler, that can take several different forms, such as the power splitter, the tap coupler, the star coupler or the power combiner. All these components essentially take several multimode fibers and bundle them together by either mechanically holding them or twisting them together, and the structure is fused and/or tapered in order to induce coupling between the fibers from the input to the output. The basic description of this coupling is given in U.S. Pat. No. 4,291,940 of Kawasaki et al. It discloses that if two multimode fibers are placed side by side and then fused together using a heat source, there is some optical power transfer from one fiber to the other. Such transfer can be increased as the structure is pulled and tapered.
This basic fused-tapered concept was used in several subsequent patents such as U.S. Pat. Nos. 4,392,712 and 4,330,170 where it became apparent that this procedure could also be used for more than two fibers, thus creating M×N fused taper bundles, where M is the input number of fibers and N is the output number of fibers. Moreover, the fuse and taper process received some further improvements such as described in U.S. Pat. Nos. 4,426,215 and 4,550,974 where several techniques are disclosed to improve the uniformity of the power distribution in the fused-tapered multimode fiber bundles. In particular, U.S. Pat. No. 4,550,974 describes a process presently known in the art as the “cut and fuse” process where a fused tapered multimode fiber bundle is cut and then fused together again to produce a better mode scrambling effect and thus better uniformity. From this process, it quickly became apparent that one did not need to fuse the same two coupler halves together, but one could put together two different coupler halves, thereby creating another way of making M×N couplers.
As applications of multimode fiber evolved, there came another application that can benefit from this process. The double clad fiber amplifiers or lasers use a type of fiber, the double clad fiber (DCF), that has a single-mode core doped with rare-earth ions, such as ytterbium, erbium or neodinium, that is surrounded by an optical cladding of far larger diameter. This cladding is a highly multimode waveguide and it is surrounded by another optical cladding having a lower refractive index, which may be a polymer cladding. To amplify an optical signal propagating through the DCF core, one needs to optically pump the rare-earth ions. This pump optical power can be injected in the core in the same manner as in single-mode fiber amplifiers, but the purpose of the double clad is that the pump power can be injected into the inner cladding which surrounds the core. Because some of the cladding modes travel through the core, they provide energy to the rare-earth ions and enable the amplification of the signal to occur. Moreover, because the inner cladding is far larger than the core, it is possible to input a greater number of pump laser light and spatially multiplex the same in the cladding, rather than wavelength or polarization multiplex the pump laser in the core. Thus, a much greater amount of pump power is available in DCF for the amplification than in single-mode fiber amplifiers.
In some DCF amplifiers or lasers, the coupling is achieved by bulk optics, coupling the pump power through lens and mirrors into the double cladding. U.S. Pat. No. 5,864,644 describes how this can be done with a multimode taper bundle using a similar approach as the “cut and fuse” technique, where the second coupler half is replaced by a DCF. The patent also describes how it is possible to include in the bundle s single-mode fiber, that will connect to the single-mode core of the DCF, thus allowing a signal to go through the coupler and be amplified or reversely, if the coupler is used in a counter pump configuration (i.e. the pump power and the signal go in the opposite direction), to let the signal out of the amplifier with minimum loss. A modification to this structure is disclosed in U.S. Pat. No. 6,434,302, where it is stated that for better performance, the tapered bundle and DCF structure must be tapered further than the diameter of the DCF to improve mode distribution for improved gain efficiency.
In high power amplifiers and lasers, as the power available for pump is greater, the power output of the amplifier or laser is also larger, to the point where the light intensity in the doped glass becomes large enough to damage the glass or to produce undesirable non-linear effects, such as Raman or Brilloin scattering. Thus, a new generation of DCF fibers has been developed to address these high power situations. These fibers have a large core area so that, even if the power is high, the intensity in the core remains reasonable. Even if one decreases the index step of the core waveguide, the large core is not necessarily single-mode at the laser wavelength. The fiber core is few-moded. One must carefully excite the core fundamental mode to have the amplification in that mode, that will produce the best output beam. This problem is not addressed in U.S. Pat. Nos. 5,864,644 and 6,434,302 which deal only with a single-mode connection. A single-mode connection is simple because one cannot excite anything other than the fundamental mode in the connection, even if the splice between the tapered fiber bundle and the DCF is bad. In the few-mode case, this connection is crucial to the proper functioning of the amplifier.
Thus, there is a need for a coupler that provides a connection of a bundle of multimode pump fibers that have a few-mode signal fiber in their centre, to a large area core double clad fiber (LACDCF).